Chemerin, a chemoattractant protein and adipokine, has been identified as the endogenous ligand for a G protein–coupled receptor encoded by the gene CMKLR1 (also known as ChemR23), and as a consequence the receptor protein was renamed the chemerin receptor in 2013. Since then, chemerin has been identified as the endogenous ligand for a second G protein–coupled receptor, encoded by the gene GPR1. Therefore, the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification recommends that the official name of the receptor protein for chemokine-like receptor 1 (CMKLR1) is chemerin receptor 1, and G protein–coupled receptor 1 is chemerin receptor 2 to follow the convention of naming the receptor protein after the endogenous ligand. Chemerin receptor 1 and chemerin receptor 2 can be abbreviated to Chemerin1 and Chemerin2, respectively. Chemerin requires C-terminal processing for activity, and human chemerin21–157 is reported to be the most active form, with peptide fragments derived from the C terminus biologically active at both receptors. Small-molecule antagonist, CCX832, selectively blocks CMKLR1, and resolvin E1 activation of CMKLR1 is discussed. Activation of both receptors by chemerin is via coupling to Gi/o, causing inhibition of adenylyl cyclase and increased Ca2+ flux. Receptors and ligand are widely expressed in humans, rats, and mice, and both receptors share ∼80% identity across these species. CMKLR1 knockout mice highlight the role of this receptor in inflammation and obesity, and similarly, GPR1 knockout mice exhibit glucose intolerance. In addition, the chemerin receptors have been implicated in cardiovascular disease, cancer, steroidogenesis, human immunodeficiency virus replication, and neurogenerative disease.

BackgroundCirculating levels of chemerin are significantly higher in hypertensive patients and positively correlate with blood pressure. Chemerin activates chemokine‐like receptor 1 (CMKLR1 or ChemR23) and is proposed to activate the “orphan” G‐protein‐coupled receptor 1 (GPR1), which has been linked with hypertension. Our aim was to localize chemerin, CMKLR1, and GPR1 in the human vasculature and determine whether 1 or both of these receptors mediate vasoconstriction.Methods and ResultsUsing immunohistochemistry and molecular biology in conduit arteries and veins and resistance vessels, we localized chemerin to endothelium, smooth muscle, and adventitia and found that CMKLR1 and GPR1 were widely expressed in smooth muscle. C9 (chemerin149–157) contracted human saphenous vein (pD 2=7.30±0.31) and resistance arteries (pD 2=7.05±0.54) and increased blood pressure in rats by 9.1±1.0 mm Hg at 200 nmol. Crucially, these in vitro and in vivo vascular actions were blocked by CCX832, which we confirmed to be highly selective for CMKLR1 over GPR1. C9 inhibited cAMP accumulation in human aortic smooth muscle cells and preconstricted rat aorta, consistent with the observed vasoconstrictor action. Downstream signaling was explored further and, compared to chemerin, C9 showed a bias factor=≈5000 for the Gi protein pathway, suggesting that CMKLR1 exhibits biased agonism.ConclusionsOur data suggest that chemerin acts at CMKLR1, but not GPR1, to increase blood pressure. Chemerin has an established detrimental role in metabolic syndrome, and these direct vascular actions may contribute to hypertension, an additional risk factor for cardiovascular disease. This study provides proof of principle for the therapeutic potential of selective CMKLR1 antagonists.

Protease-activated receptor-2 (PAR2) has been implicated in multiple pathophysiologies but drug discovery is challenging due to low small molecule tractability and a complex activation mechanism. Here we report the pharmacological profiling of a potent new agonist, suggested by molecular modelling to bind in the putative orthosteric site, and two novel PAR2 antagonists with distinctly different mechanisms of inhibition. We identify coupling between different PAR2 binding sites. One antagonist is a competitive inhibitor that binds to the orthosteric site, while a second antagonist is a negative allosteric modulator that binds at a remote site. The allosteric modulator shows probe dependence, more effectively inhibiting peptide than protease activation of PAR2 signalling. Importantly, both antagonists are active in vivo, inhibiting PAR2 agonist-induced acute paw inflammation in rats and preventing activation of mast cells and neutrophils. These results highlight two distinct mechanisms of inhibition that potentially could be targeted for future development of drugs that modulate PAR2.

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid metabolite, plays a critical role in the orchestration of immune responses. S1P levels within the mammalian host are tightly regulated, in part through the activity of S1P lyase (S1PL) which catalyses its irreversible degradation. Herein, we describe the identification and characterization of secreted S1PL orthologues encoded by the facultative intracellular bacteria Burkholderia pseudomallei and Burkholderia thailandensis. These bacterial orthologues exhibited S1PL enzymatic activity, functionally complemented an S1PL-deficient yeast strain and conferred resistance to the antimicrobial sphingolipid D-erythro-sphingosine. We report that secretion of these bacterial S1PLs is pH-dependent, and is observed during intracellular infection. S1PL-deficient mutants displayed impaired intracellular replication in murine macrophages (associated with an inability to evade the maturing phagosome) and were significantly attenuated in murine and larval infection models. Furthermore, treatment of Burkholderia-infected macrophages with either S1P or a selective agonist of S1P receptor 1 enhanced bacterial colocalisation with LAMP-1 and reduced their intracellular survival. In summary, our studies confirm bacterial-encoded S1PL as a critical virulence determinant of B. pseudomallei and B. thailandensis, further highlighting the pivotal role of S1P in host-pathogen interactions. In addition, our data suggest that S1P pathway modulators have potential for the treatment of intracellular infection.

Protease-activated receptor 2 (PAR2) is a G-protein-coupled receptor that is activated by proteolytic cleavage of its N-terminus. The unmasked N-terminal peptide then binds to the transmembrane bundle, leading to activation of intracellular signaling pathways associated with inflammation and cancer. Recently determined crystal structures have revealed binding sites of PAR2 antagonists, but the binding mode of the peptide agonist remains unknown. In order to generate a model of PAR2 in complex with peptide SLIGKV, corresponding to the trypsin-exposed tethered ligand, the orthosteric binding site was probed by iterative combinations of receptor mutagenesis, agonist ligand modifications, and data-driven structural modeling. Flexible-receptor docking identified a conserved binding mode for agonists related to the endogenous ligand that was consistent with the experimental data and allowed synthesis of a novel peptide (1-benzyl-1H[1,2,3]triazole-4-yl-LIGKV) with functional potency higher than that of SLIGKV. The final model may be used to understand the structural basis of PAR2 activation and in virtual screens to identify novel agonists and competitive antagonists. The combined experimental and computational approach to characterize agonist binding to PAR2 can be extended to study the many other G protein-coupled receptors that recognize peptides or proteins.

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Sphingolipids (SLs) are ubiquitous elements in eukaryotic membranes and are also found in some bacterial and viral species. As well as playing an integral structural role, SLs also act as potent signaling molecules involved in numerous cellular pathways and have been linked to many human diseases. A central SL signaling molecule is sphingosine-1-phosphate (S1P), whose breakdown is catalyzed by S1P lyase (S1PL), a pyridoxal 5′-phosphate (PLP)-dependent enzyme that catalyzes the cleavage of S1P to (2E)-hexadecenal (2E-HEX) and phosphoethanolamine. Here, we show that the pathogenic bacterium, Burkholderia pseudomallei K96243, encodes two homologous proteins (S1PL2021 and S1PL2025) that display moderate sequence identity to known eukaryotic and prokaryotic S1PLs. Using an established MS-based methodology, we show that recombinant S1PL2021 is catalytically active. We also used recombinant human fatty aldehyde dehydrogenase to develop a spectrophotometric enzyme-coupled assay to detect 2E-HEX formation and measure the kinetic constants of the two B. pseudomallei S1PL isoforms. Furthermore, we determined the X-ray crystal structure of the PLP-bound form of S1PL2021 at 2.1 Å resolution revealing that the enzyme displays a conserved structural fold and active site architecture comparable with known S1PLs. The combined data suggest that B. pseudomallei has the potential to degrade host SLs in a S1PL-dependent manner.

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